Organic acids prepared from saccharides such as starch, sucrose, or glucose, or from n-paraffines by fermentation include lactic, gluconic, kojic, citric, succinic, malic, and itaconic acids. In the process for preparing them, the step of fermentation is relatively simple, but the steps of recovery and purification of a product, an organic acid, are complicated with poor efficiency. For example, recovery of a representative organic acid, lactic acid, is made as follows. A cell body is filtered out of a fermentation liquor after the reaction has been completed. For increasing the filtration efficiency, it is necessary to coagulate a cell body and dissolved proteins in the filtrate by heating. Subsequently, sulfuric acid is added to the filtrate to precipitate an alkali, such as calcium carbonate or calcium hydroxide, which was added to the fermentation liquor to adjust the pH thereof, as a sulfate, followed by filtration. The filtrate is then freed from impurities, such as unreacted raw material, with an activated charcoal. The resulting solution is subjected to, for example, purification with an ion exchange resin, distillation in the form of a corresponding methyl ester followed by hydrolysis, or extraction with a solvent followed by extraction with water to obtain a free acid.
To improve the above-mentioned process involving a number of complicated steps, there has been proposed a process for continuously conducting fermentation by continuously recovering a formed organic acid from a fermentation liquor according to electrodialysis using an ion exchange membrane. More specifically, Japanese Patent Publication No. 50958/1981 discloses a continuous fermentation process comprising discharging a fermentation liquor from an itaconic acid fermenter at a constant rate, filtering a cell body, removing high molecular weight impurities by ultrafiltration, converting itaconic acid into an alkali metal salt thereof, and supplying the salt to an electrodialysis stack for separation of the itaconate from the fermentation liquor while returning the liquor form which the itaconate has been removed to the fermenter after addition of a substrate thereto. The disclosure mentions advantages that accumulation of fermentation-inhibiting substances in the fermentation system can be prevented by continuously withdrawing the product to enable the fermentation to be continuously carried out with high efficiency, and that the withdrawn itaconate is so pure that it can be readily separated by precipitation. On the other hand, Japanese Patent Publication No. 32959/1983 discloses a process comprising removing a cell body from a fermentation liquor of glutamic acid, converting the glutamic acid into a salt with an alkali, and recovering the salt by electrodialysis.